This invention relates to a method of coating a liquified coating compound on a belt-shaped flexible support (hereinafter referred to as "a web" when applicable).
One of the extensively employed methods known in the prior art of coating a liquified coating compound (hereinafter referred to as "a coating liquid" when applicable) on a web is a multi-layer slide bead coating method disclosed by U.S. Pat. No. 2,761,791 (Russell et al). In this method, a plurality of coating liquids running along a plurality of slopes are coated on a running web through a bead formed at the position where the liquids meet the web. In the method, it is essential to maintain the bead stable to satisfactorily coat the liquids on the web. However, when the coating rate is increased, it is difficult to maintain the bead stable.
U.S. Pat. No. 2,681,294 (Beguin et al) teaches that the bead can be advantageously maintained stable by creating a negative pressure on one side of the bead (upstream as viewed in the direction of advancement of the web) to provide a difference between the pressures on the two surfaces of the bead. However, the instability of the bead cannot be eliminated merely by solving the problems concerning the direction of advancement of the web and the pressure difference between the upper and lower surfaces of the bead. That is, the bead cannot be maintained stable without also solving the problem of three-dimensional balance including the stability of the bead in the direction of the width of the bead (same as the direction of the width of the web). According to the experiments made by the inventors, breakage of the bead begins at the marginal sides thereof in many cases. Accordingly, this problem must be solved before the coating rate is increased.